In recent years, to achieve high data transmission capacity in fiber-optic transmission systems, research studies for increasing a modulation rate and improving modulation schemes (including multi-level modulation and polarization division multiplexing) are being made. With dual polarization quadrature phase-shift keying (DP-QPSK) using a digital coherent technique, long distance fiber-optic transmission at 100 GB/s has been achieved. Polarization division multiplexing (PDM) permits two independent data signals to be transmitted at a time by combining two orthogonally polarized light signals.
FIG. 1 illustrates a typical DP-QPSK modulator module 100. The modulator module 100 includes a modulator chip 120 with four Mach-Zehnder (MZ) interferometers 121-124 arranged in parallel, a polarization rotator (PR) 125, and a polarization beam combiner (PBC) 126. A light beam emitted from a light source 105 such as a laser diode (LD) is input to the modulator chip 120 via a lens 101. The input light is modulated under application of different electrical signals to the MZ interferometers 121-124. The direction of polarization of the light component output from one of the MZ interferometer pairs is rotated by 90 degrees at the PR 125 with respect to the light component output from the other pair of the MZ interferometers. These light components are combined at the PBC 126. The resultant signal is a multi-level modulated and polarization division multiplexed signal. The signal light output from the modulator module 100 is amplified by an erbium-doped fiber amplifier (EDFA) 127 and undergoes noise reduction at a bandpass filter (BPS) 128. Then, the signal is output to a transmission path.
With the configuration of FIG. 1, PDM and optical amplification are performed at separate components and therefore, the transmitter size becomes larger. To achieve a compact transmitter structure, the configuration of FIG. 2 may be provided in which the function of optical amplification is incorporated, together with the PR 125 and PBC 126 used for PDM, in a modulator module 200. In order to put the function of optical amplification into the modulator module 200, a semiconductor optical amplifier (SOA) 227 is used in place of the EDFA 127. The PBC 126, the SOA 227 and the BPF 228 are optically coupled using lenses 204 and 206.
The publications listed below are also known.    [Patent Document 1] Japanese Laid-open Patent Publication No. 2011-188213    [Patent Document 2] Japanese Laid-open Patent Publication No. 2009-60461